Multi-chamber carburizing apparatus

ABSTRACT

The multi-chamber carburizing apparatus results in a continuous process including a heating furnace having inlet door means through which, when open, work is fed into the furnace, a carburizing furnace connected thereto, interconnecting double door means between the heating furnace and the carburizing furnace through which, when open, work is fed from the heating furnace into the carburizing furnace, a diffusion furnace, interconnecting double door means between the carburizing furnace and diffusion furnace through which, when open, work is fed from the carburizing furnace into the diffusion furnace, and a cooling chamber which may be a water cooled section or an oil quench tank having inlet door means through which, when open, work is fed from the diffusion furnace into the cooling chamber, or quench tank. The double door means have a space therebetween which may be filled with a neutral or other gas to act as a barrier between the furnaces. Gas passageways are provided beneath the various door means for flow of gas from the various furnaces at a controlled rate.

Schwalm et a1.

[ 1 May 16, 1972 MULTI-CHAMBER CARBURIZING APPARATUS Int. C

Inventors:

Assignee:

Field of Search Donald J. Schwalm, Northville; Edward C. Bayer, Dearborn, both of Mich.

Holcroft & Company, Livonia, Mich.

Mar. 23, 1970 US. Cl. ..266/4 A, l48/l6.5, 263/36 1. ..C2ld 1/78 148/l6.5, 263/36; 266/4 A,

266/4 R, 4 F, 4 B

References Cited UNITED STATES PATENTS Primary ExaminerGerald A. Dost Attorney-Whittemore, Hulbert & Belknap [57] ABSTRACT The multi-chamber carburizing apparatus results in a continuous process including a heating furnace having inlet door means through which, when open, work is fed into the furnace, a carburizing furnace connected thereto, interconnecting double door means between the heating furnace and the carburizing furnace through which, when open, work is fed from the heating furnace into the carburizing furnace, a diffusion furnace, interconnecting double door means between the carburizing furnace and diffusion furnace through which, when open, work is fed from the carburizing furnace into the diffusion furnace, and a cooling chamber which may be a water cooled section or an oil quench tank having inlet door means through which, when open, work is fed from the diffusion furnace into the cooling chamber, or quench tank. The double door means have a space therebetween which may be filled with a neutral or other gas to act as a barrier between the furnaces. Gas passageways are provided beneath the various door means for flow of gas from the various furnaces at a controlled rate.

17 Claims, 9 Drawing Figures DIFFUSION FURNACEJ n CONTROL 11; r 6% CEfiTER 6% m. 1 1011. QUENCH 7 r NK 74 ELEVATOR CARBURIZING FURNACE Z POWER ER TRAY j4 FEEDER ELEVATOR PATENTEDMAY 16 I972 SHEET 1 [If 5 INVENTOR ATTORNEYS WIIM J PATENTEDHAY 16 I972 3, 662 996 sum 3 or 5 BY wmh /MJ FM M Q ATTORNEYS PATENTEMAY 16 I972 3. 662,996

sum 5 or 5 Fl G.8 i

A90 INVENTORS DOA/A10 so /14mm [arm/w a 15/1 n54 ATTORNEYS MULTl-CI-IAMBER CARBURIZING APPARATUS BACKGROUND OF THE INVENTION Continuous carburizing is carried out in the gas carburizing method in three stages. Firstly, the work is heated to carburizing temperature in a gaseous atmosphere which prevents scaling or de-carburization. Secondly, the work is heated in an atmosphere which has been carbon enriched and which causes carbon to penetrate into the work. The length of time which the work is exposed to the carbon enriched atmosphere controls the depth of penetration of the carbon. Thirdly, the work is subjected to a diffusion period during which the carbon content of the atmosphere is adjusted to be in equilibrium with the desired carbon content in the surface layer of the work. The diffusion period allows the high surface carbon which is induced during the carburizing period to diffuse and produce a satisfactory case upon the work.

In previously known continuous carburizing furnaces, all three of these stages have generally been carried out in what amounts to a single chamber. Such chambers have usually been elongated structures wherein the work has been fed in at one end and removed at the other. Such chambers are usually full of work requiring inlet and outlet doors at the ends thereof to be open at the same time to enable simultaneous feeding of work into the chamber and removal of finished work at the other end.

It has been difficult in such furnaces to precisely determine the boundaries of the heating, carburizing, and diffusing zones. Because the degree of carburization depends on the time which the work is in the carburizing gas atmosphere and because this is determined in turn by the rate of speed of work through the chamber, migration of the carburizing atmosphere from the carburizing zone has an effect on the rate at which carburization is effected. This makes it difficult to control the carburizing depth in components of different sections passing through the furnace and even in different portions of the same component.

Prior furnaces are controlled by controlling the carbon enrichment of the atmosphere in the various sections of the furnace. Because the atmosphere can migrate from one section to another, it has been difficult to maintain entirely the desired atmosphere in each section. It has therefore been necessary, to some extent, to operate such furnaces on a trial and error basis.

In accordance with the present invention, each section of the apparatus, namely, the heating furnace, the carburizing furnace and diffusion furnace are completely separate chambers. The inlet to the heating furnace and the outlet of the diffusion furnace are protected from the atmosphere by single door structures usually referred to as vestibules. Archways are provided between the heating furnace and carburizing furnace and carburizing furnace and diffusion furnace. The archways are normally closed by means of double door structures which are effective in isolating the atmospheres of the furnaces from each other.

SUMMARY OF THE INVENTION The multi-chamber carburizing apparatus for continuous carburization includes a heating furnace having inlet door means through which, when open, work is fed into the heating furnace. A carburizing furnace is connected to the heating furnace. lnterconnecting double door means are provided between the heating furnace and the carburizing furnace through which, when open, work is fed from the heating furnace into the carburizing furnace. A diffusion furnace is connected to the carburizing furnace. lnterconnecting double door means are provided between the carburizing furnace and the diffusion furnace through which, when open, work is fed from the carburizing furnace into the diffusion furnace. A cooling chamber or oil quench tank is connected to the diffusion furnace. lnterconnecting door means are provided between the diffusion furnace and the cooling chamber or quench tank through which, when open, work is fed from the diffusion furnace into the cooling chamber. The double door means have a space therebetween which may be filled with a neutral or other gas to act as a barrier between the furnaces. Gas passageways are provided beneath the various door means for flow of gas from the various furnaces at a controlled rate.

DRAWINGS FIG. 1 is a diagrammatic view illustrating one embodiment of a multi-chamber carburizing apparatus in accordance with the present invention;

FIG. 2 is an elevational view in section of the double door arrangement between the heating furnace and carburizing furnace taken substantially along the line 2-2 of FIG. 3 looking in the direction of the arrows;

FIG. 3 is a sectional view of the double door taken substantially along the line 33 of FIG. 2 looking in the direction of the arrows;

FIG. 4 is an elevational view in section of the single door provided at the entrance to the heating furnace;

FIG. 5 is a sectional view taken substantially along the line 5-5 of FIG. 4 looking in the direction of the arrows;

FIG. 6 is an elevational view in section of the single door at the exist of the diffusion furnace leading to an oil quench tank;

FIG. 7 is a longitudinal sectional view of the carburizing furnace;

FIG. 8 is a longitudinal sectional view of the heating furnace; and

FIG. 9 is a longitudinal sectional view of the diffusion furnace.

PREFERRED EMBODIMENT FIG. 1 illustrates the general layout of the multi-chamber carburizing apparatus 10. The apparatus 10 includes an elevator structure 12 which lifts trays loaded with work to be carburized from a loading station to a powered tray feeder conveyor 14. The conveyor 14 carries the trays to a charging vestibule 16 where they are deposited through an alligator door 17 for movement into a heating furnace 18. The work is supportedin jigs and the jigs are mounted on the trays. The jigs are so arranged that the whole of the surface of the work which is required to be carburized is open to the atmosphere in the various furnace cambers. The trays are moved from the vestibule 16 into the heating furnace 18 as indicated by the arrow 20 by means of motor driven pusher structure 22. An inlet door 24, which is normally closed, is raised when it is desired to move a tray from the vestibule 16 into heating furnace 18. The work is heated to carburizing temperature in the heating furnace 18 in a gaseous atmosphere which prevents scaling or decarburization. The trays are pushed along rails within the heating furnace in two rows as indicated by arrows 26, 28 by means of motor driven pusher structure 30.

The heating furnace is connected to a carburizing furnace 32 by an archway 34. The archway is normally closed by means of a double door structure 36 which is raised when it is desired to move loaded trays from the heating furnace to the carburizing furnace in the direction of arrow 38. Motor driven pusher apparatus 40 is provided to drive the trays from the heating furnace into the carburizing furnace.

The work trays are again pushed through the carburizing furnace on rails as indicated by the arrows 42, 44 by means of motor driven pusher structure 46. A neutral barrier gaseous atmosphere may be maintained in the space 48 between the double doors 50, 52 to act as a barrier between the atmospheres in the carburizing furnace and the heating furnace. The atmosphere in the carburizing furnace is carbon enriched and maintained at an elevated temperature to cause carbon to penetrate into the work. The length of time which the work is exposed to the carbon enriched atmosphere controls the depth of penetration of the carbon.

A diffusion furnace 54 is connected to the outlet end of the carburizing furnace 32 by an archway 56. A second double door structure 58 is provided between the carburizing furnace and diffusion furnace. This structure is in all significant respects identical to the double door structure 36. When the double door structure 58 is raised, a motor driven pusher structure 60 is actuated to push two trays into the diffusion furnace in the direction of the arrow 62. The work is subjected to an elevated temperature in the diffusion furnace in an atmosphere which is adjusted to the desired carbon content in the surface of the work. While the work is in the diffusion furnace, the high surface carbon which is produced during the carburizing period is diffused to produce a case upon the work which has the required carbon gradient from the interior of the case to the external surface thereof.

Two rows of trays are pushed through the diffusion furnace 54 in the direction of the arrows 64, 66, by means of a motor driven pusher structure 68. The diffusion furnace is connected to an oil quench tank 70. A discharge door 72 is provided between the diffusion furnace and oil quench tank. When the door 72 is raised, the work is pushed into the first chamber 74 of the tank 70'by means of motor driven pusher structure 76. The oil quench tank is filled with oil and comprises two vertical chambers 74, 78 which are separated at their upper portions by means of a wall 80. The wall 80 does not extend to the bottom of the oil quench tank but does extend beneath the level of oil therein thus forming an oil seal between the two vestibules. Thus, the chambers 74, 78 are sealed from each other by the oil in the tank. The work is dropped into the oil in chamber 74 by means of an elevator structure and is then moved sidewardly beneath the wall 80 and thence upwardly by the elevator structure where it is moved out of the oil quench tank through an opening 82 in the direction of arrow 84 by means of a powered pusher structure 86. It is not necessary to provide a door at the opening 82 because, at this point, the atmosphere of the carburizing apparatus need no longer be controlled.

The work trays are carried from the oil quench tank on a conveyor structure 88 to an elevator 90. The elevator 90 carries the work downwardly to an unloading station. The empty trays are placed on a conveyor 92 and fed through a gate 94 back to the starting point represented by the elevator 12.

The entire operation of the carburizing apparatus is controlled by means of a control center 96 which includes conventional timer structure, switching apparatus, and control means to actuate the various pusher mechanisms cause raising and lowering of the doors in proper sequence, and control the temperature and atmosphere of the furnaces. In a typical apparatus, the work may remain in the heating furnace 18 for 2.67 hours, in the carburizing furnace 32 for 6.67 hours and in the diffusion furnace 54 for 2.00 hours. The capacity of such a furnace may be in the neighborhood of 1,350 pounds of work per hour which would amount to about three trays per hour. The case depth may be from 0.060 to 0.080 inch. The tray size may be 22 by 22 inches and weigh 42 pounds. Baskets may be used to contain the work which are about inches square and 10 inches high and weigh about 38 pounds and are placed on the trays.

Referring now more specifically to the structure of the various chambers with particular emphasis on the various gas inlets and vent lines and the mechanical structure of the various doors, reference is first made to FIGS. 4, 5, and 8. As previously mentioned, the loaded trays are first received in the vestibule 16 and are placed in alignment with the door 24. Upon raising of the door 24, the trays are pushed into the heating furnace 18 by means of the pusher structure 30. The heating furnace 18 includes supporting framework 98 which is lined with refractory material 99 to define a heating chamber. A number of pairs of radiant tube burners 102, some of which may be positioned above and some below the work, are provided for heating of the work. The work is arranged in two rows, each of the rows being supported on one of the set of tracks 104, 106. The area between the tracks is depressed and is lined with refractory material. The trays slide along the tracks through the furnace, being fed therethrough by means of pusher structures 22, 30, 40. The atmosphere in the heating furnace 18 comprises a neutral gas which will not cause scal ing or decarburization.

The inlet opening 108 of the heating furnace 18 is normally closed by the door 24, which comprise a support frame 110 lined with refractory material 112. A bracket 114 is attached to the upper edge of the door. A chain 116 extends from the bracket 114 around a sprocket 118 which is positioned thereabove. The sprocket 118 is power driven through a belt and pulley arrangement 120 to case raising of the door as required. The door, because of its weight, will close as a result of gravity.

As before noted, there are a series of depressions between the tracks 104, 106. All but the depression 122 are plugged beneath the door. The depression 122 serves as a passageway to permit gas from the furnace to flow at a controlled rate into the vestibule 16. Gas is vented from the vestibule by means of a vent line 124. Fresh gas is supplied to the upper portion of the heating furnace via tubes 126, 128 which extend through end walls 130. 132.

After the trays of work have passed through the heating furnace 18, they are, as previously mentioned, moved to the carburizing furnace 32 by means of the pusher structure 40. The structure of the carburizing furnace and double doors between the carburizing furnace and heating furnace are best illustrated in FIGS. 2, 3, and 7. Only the double door 36 is specifically illustrated, the other double door 58 being in all pertinent respects identical thereto. As will be noted, the carburizing furnace 32 is approximately three times as long as the heating furnace 18 because the time required in this furnace is approximately three times that required in the heating furnace.

The carburizing furnace 32 is constructed in a similar manner to the heating furnace 18 having a steel framework 134 with walls formed of refractory material 136. Dual tracks 138, are provided for the two rows of trays to be slid over. A plurality of radiant tube burners 142 are provided above and below the work for heating as previously described. A gas inlet tube 144, 146 is provided in each end wall 148, 150 for maintaining the proper carburizing atmosphere'within the furnace 32. Vent openings 147, 149 are provided in the lower portion of the chamber. 1

As best illustrated in FIGS. 2 and 3, the double door structure 36 comprises the pair of door elements 50, 52 which are spaced apart to define the interior chamber 48. The door elements are pivotally connected at their upper ends to a crossbar structure 158. An upstanding link extends upwardly from the crossbar structure into the verticalchamber 162. An opening 164 is provided in the top of the chamber 162. A chamber 166 having vertical portion 168 and enlarged upper portion 170 extends from the opening 164. A motor driven sprocket 172 is provided in the portion 170 and is attached to a chain 174 which extends into connection with the upper end of link 160. When the sprocket 172 is driven, the double door structure is lifted to permit movement of trays through the archway 34 from the heating furnace 18 into the carburizing furnace 32. The double door structure is lowered by gravity.

The lower ends of the door elements 50, 52 are received in notches 176, 178 provided in rails of refractory material 180, 182 provided in the lower comers of the chamber defined by the archway 34. A gas opening 184 is provided in the side wall 186 of the archway 34. The opening 184 may be used either as a gas inlet or a gas vent line depending on the system of gas introduction used. When used as a gas inlet, the incoming gas may be of a neutral nature so as to provide a neutral gaseous barrier between the heating furnace 18 and the carburizing furnace 32.

As previously mentioned in connection with the door 24, all of the spaces between the tracks 138, 140 are plugged beneath the door elements excepting the center space which forms a slot 188 for gas passage in either direction between the furnaces 18, 32.

After the work loaded trays have been in the carburizing furnace for a sufficient time, they are moved out of the carburizing furnace through the archway 56 and through the double doors 58 into the diffusion furnace 54, the double doors 58 being constructed as described in connection with the double doors 36, and also having a gas passageway 187 and gas opening 189 (FIG. 1).

The diffusion furnace 54 and associated oil quench tank structure 70 are best illustrated in FIGS. 6 and 9. The diffusion furnace is constructed in the manner previously described, and includes a steel framework 190 having interior walls formed of refractory material 192. Dual tracks 194, 196 are provided for movement of work loaded trays. A plurality of radiant tube burners 198 are positioned to properly heat the interior of the diffusion furnace 54. Work loaded trays are moved through the diffusion furnace 54 by means of the pusher structures 60, 68, 76.

The single door 72 at the outlet 200 from the diffusion furnace 54 is constructed and actuated in the same manner as the door 24 previously described. The center slot 202 beneath the door 72 is not plugged to provide a passageway for gas from the diffusion furnace to flow into the chamber 74 which defines a first discharge vestibule, for purging of the diffusion furnace 54 at a controlled rate. A vent line 204 is provided in the wall 80 for venting the chamber 74. As previously mentioned, there is no gaseous communication between the chambers 74, 78 and thus purging of the chamber 74 is maintained at a minimum because this vestibule is not exposed to outside air. As will be noted in FIG. 9, a gas inlet tube 206 is provided for maintaining the atmosphere within the furnace 54 at the desired consistency.

Work loaded trays are received from the diffusion furnace 54 on an elevator structure 208. When the elevator structure 208 is loaded, it is caused to descend into oil 210 for the quenching operation. The elevator structure 208 carries the work loaded trays beneath the lower edge 212 of the wall 80 whence it is moved sidewardly to the left as viewed in FIG. 6 and then upwardly for unloading through the opening 82 by means of the pusher structure 86.

There are basically three methods in which the various gas inlet and vent openings may be utilized to control the atmosphere in the carburizing apparatus. In each of the methods, the inlets 126, 128 to the heating furnace 18 are utilized to introduce neutral gas into the furnace chamber. The inlets 144, 146 to the carburizing furnace 32 are always utilized as a means for introducing generator gas and hydrocarbon gas into the carburizing furnace. The inlet 206 to the diffusion furnace 54 is utilized as a means for introducing generator gas and combustion products or air into the diffusion furnace. The vent line 124 in the vestibule 16 is utilized, in conjunction with the passageway 122 under door 24, as a vent line for the vestibule 16 for gases which flow from the heating furnace 18. Similarly, the vent line 204 from the discharge vestibule 74 is utilized, in conjunction with the passageway 202 under door 72, as a vent line for the discharge vestibule 74 for gases which flow from the diffusion furnace 54.

The variations in controlling the atmosphere within the carburizing apparatus revolve about the use of the gas lines 184, 189 which extend from the spaces between the double doors 36, 58 and the gas lines which extend from the openings 147, 149 in the carburizing furnace 32.-

In the first case, neutral gas is introduced through inlets 184, 189 to create a barrier between the heating furnace 18 and carburizing furnace 32 and between carburizing furnace 32 and diffusion furnace 54. The openings 147, 149 are then utilized as vent means for the carburizing furnace 32.

In the second case, the gas line 184 between the double doors 36 is utilized as a vent line between the carburizing furnace and heating furnace, the passageway 188 providing for communication to the interior space 48 between the door elements 50, 52. The gas line 189 between the double door structure 58 is utilized as a vent line between the carburizing furnace 32 and the diffusion furnace 54, the passageway 202 providing the desired fluid communication.

Finally generator gas may be introduced through gas line 184 between the door elements of double door structure 36 to form a gaseous barrier between the carburizing furnace 32 and heating furnace 18 while the gas line 189 between the door elements of double door structure 58 is again used as a vent line between the carburizing furnace 32 and the diffusion furnace 54.

The three variations of introduction of the several types of gases for carrying out the carburizing operation in the multichamber carburizer are more explicitely as follows:

EXAMPLE 1 a. Introduce neutral gas into heating furnace 18 through inlets 126, 128.

b. Introduce neutral gas through inlets 184, 189 to create a gaseous barrier in double doors 34, 56.

c. Introduce generator gas plus hydrocarbon gas through inlets 144, 146 into carburizing furnace 32.

d. Excess gas from the carburizing furnace is vented through outlets 147, 149. I

e. Introduce generator gas plus combustion products or air through 206 to produce an atmosphere in diffusion furnace 54, which is in equilibrium with the desired surface carbon.

f. 204 is the vent line for the discharge vestibule.

g. 124 is the vent line for the charge vestibule.

EXAMPLE 2 a. Introduce neutral gas through 126, 128 into the heating furnace 18.

b. Introduce generator gas and hydrocarbon gas through 144, 146 into carburizing furnace 32.

c. Introduce generator gas and combustion products or air through 206 into diffusion furnace 54.

d. 124 is the vent line for charge vestibule.

e. 204 is the vent line for discharge vestibule.

f. The vent line between carburizing furnace 32 and heating furnace 18 is 184.

g. The vent line between carburizing furnace 32 and diffusion furnace 54 is 189.

EXAMPLE 3 a. Introduce neutral gas 126, 128 into heating furnace 18.

b. Introduce generator gas and hydrocarbon gas through 144, 146 into carburizing furnace 32.

c. Introduce generator gas and combustion products or air through 206 into diffusion furnace 54.

d. The vent line for charge vestibule is 124.

e. The vent line for discharge vestibule is 204.

f. The vent line between charging furnace 32 and diffusion furnace 54 is 189.

g. Introduce generator gas at 184 to form atmospheric barrier between carburizing furnace 32 and hating furnace 18.

We claim:

1. A multi-chamber carburizing apparatus for use in a continuous carburizing process comprising a heating furnace having inlet door means through which, when open, work is fed into the furnace, a carburizing furnace connected thereto, interconnecting door means between the heating furnace and the carburizing furnace through which, when open, work is fed from the heating furnace into the carburizing furnace, and a diffusion furnace connected to the carburizing furnace, interconnecting door means between the carburizing furnace and diffusion furnace through which, when open, work is fed from the carburizing furnace into the diffusion furnace, and outlet door means on the diffusion furnace through which, when open, work is fed from the diffusion furnace, said apparatus being further characterized in'that at least one of said interconnecting door means is provided with a space for a barrier gaseous atmosphere.

2. Apparatus as defined in claim 1, further characterized in that each of said interconnecting door means adjacent the carburizing furnace is a double door structure having two laterally spaced door elements defining therebetween a space for a barrier gaseous atmosphere.

3. Apparatus as defined in claim 2 wherein each double door structure is provided with opening means communicating with said space for utilization alternatively as a gas inlet or a gas outlet.

4. Apparatus as defined in claim 3 wherein there is a gas passageway beneath each double door structure for communicating between the respective furnaces and the space between the door elements.

5. Apparatus as defined in claim 4, further characterized in the provision of a closed charging vestibule connected to the heating furnace and a closed discharge vestibule connected to the diffusion furnace for, respectively, receiving work for feeding into the heating furnace and for receiving finished work from the diffusion furnace, gas passageway means beneath the inlet door means for the heating furnace and the outlet door means from the diffusion furnace to provide controlled passage of gas from the furnaces to their respective vestibules, and vent means for each vestibule.

6. Apparatus as defined in claim 2 in which said door elements are pivotally connected at their upper ends to a crossbar having a vertical link within a vertical chamber for receiving said doors when raised, and means is provided operating through said link for raising and lowering the double door structure.

7. Apparatus as defined in claim 1, further characterized in the provision of another chamber connected to the outlet of the diffusion furnace, said chamber serving to cool the heated work ejected from said diffusion furnace through the aforesaid outlet door means.

8. Apparatus as defined in claim 7 wherein said cooling chamber comprises an oil quench tank, said oil quench tank being divided into a discharge vestibule directly connected to the outlet from the diffusion furnace and an outlet vestibule for discharging work from the quench tank, wall means separating said vestibules and extending into oil in the quench tank to seal the vestibules from each other.

9. Apparatus according to claim 4 in which means is provided for filling the space in at least one of said double doors with a neutral gas to create a gaseous barrier between the adjoining furnaces.

10. Apparatus according to claim 9 in which means is provided for filling with neutral gas the spaces in both of the double doors adjacent the carburizing furnace.

11. Apparatus according to claim 4 in which means is provided for introducing generator gas into the space defined by the double door between the heating furnace and the carburiz ing furnace to form a gaseous barrier.

12. The method of operating a multi-chamber carburizing apparatus which comprises transferring the work from a heating furnace into a carburizing furnace and then into a diffusion furnace, maintaining said furnaces at the optimum temperatures and atmospheric gaseous content by introducing neutral gas into the heating furnace, introducing generator gas plus hydrocarbon gas into the carburizing furnace, and introducing generator gas plus an oxidizing gaseous product into the diffusion furnace, and maintaining a barrier gas between said carburizing furnace and said heating furnace and said diffusion furnace, respectively.

13. The method according to claim 12 in which the barrier gas is a neutral gas.

14. The method according to claim 12 in which the barrier gas is maintained by venting the gas from the carburizing furnace.

15. The method according to claim 12 in which barrier gases are maintained at the entrance to the heating furnace and the exit from the diffusion furnace by venting thereinto the gases for said heating and said diffusion furnaces, respectively.

16. The method according to claim 15 in which the barrier gas adjacent the carburizing furnace is a neutral gas.

17. The method according to claim 15 wherein the barrier gas adjacent the carburizing furnace is maintained by venting the gas from said carburizing furnace. 

1. A multi-chamber carburizing apparatus for use in a continuous carburizing process comprising a heating furnace having inlet door means through which, when open, work is fed into the furnace, a carburizing furnace connected thereto, interconnecting door means between the heating furnace and the carburizing furnace through which, when open, work is fed from the heating furnace into the carburizing furnace, and a diffusion furnace connected to the carburizing furnace, interconnecting door means between the carburizing furnace and diffusion furnace through which, when open, work is fed from the carburizing furnace into the diffusion furnace, and outlet door means on the diffusion furnace through which, when open, work is fed from the diffusion furnace, said apparatus being further characterized in that at least one of said interconnecting door means is provided with a space for a barrier gaseous atmosphere.
 2. Apparatus as defined in claim 1, further characterized in that each of said interconnecting door means adjacent the carburizing furnace is a double door structure having two laterally spaced door elements defining therebetween a space for a barrier gaseous atmosphere.
 3. Apparatus as defined in claim 2 wherein each double door structure is provided with opening means communicating with said space for utilization alternatively as a gas inlet or a gas outlet.
 4. Apparatus as defined in claim 3 wherein there is a gas passageway beneath each double door structure for communicating between the respective furnaces and the space between the door elements.
 5. Apparatus as defined in claim 4, further characterized in the provision of a closed charging vestibule connected to the heating furnace and a closed discharge vestibule connected to the diffusion furnace for, respectively, receiving work for feeding into the heating furnace and for receiving finished work from the diffusion furnace, gas passageway means beneath the inlet door means for the heating furnace and the outlet door means from the diffusion furnace to provide controlled passage of gas from the furnaces to their respective vestibules, and vent means for each vestibule.
 6. Apparatus as defined in claim 2 in which said door elements are pivotally connected at their upper ends to a crossbar having a vertical link within a vertical chamber for receiving said doors when raised, and means is provided operating through said link for raising and lowering the double door structure.
 7. Apparatus as defined in claim 1, further characterized in the provision of another chamber connected to the outlet of the diffusion furnace, said chamber serving to cool the heated work ejected from said diffusion furnace through the aforesaid outlet door means.
 8. Apparatus as defined in claim 7 wherein said cooling chamber comprises an oil quench tank, said oil quench tank being divided into a discharge vestibule directly connected to the outlet from the diffusion furnace and an outlet vestibule for discharging work from the quench tank, wall means separating said vestibules and extending into oil in the quench tank to seal the vestibules from each other.
 9. Apparatus according to claim 4 in which means is provided for filling the space in at least one of said double doors with a neutral gas to create a gaseous barrier between the adjoining furnaCes.
 10. Apparatus according to claim 9 in which means is provided for filling with neutral gas the spaces in both of the double doors adjacent the carburizing furnace.
 11. Apparatus according to claim 4 in which means is provided for introducing generator gas into the space defined by the double door between the heating furnace and the carburizing furnace to form a gaseous barrier.
 12. The method of operating a multi-chamber carburizing apparatus which comprises transferring the work from a heating furnace into a carburizing furnace and then into a diffusion furnace, maintaining said furnaces at the optimum temperatures and atmospheric gaseous content by introducing neutral gas into the heating furnace, introducing generator gas plus hydrocarbon gas into the carburizing furnace, and introducing generator gas plus an oxidizing gaseous product into the diffusion furnace, and maintaining a barrier gas between said carburizing furnace and said heating furnace and said diffusion furnace, respectively.
 13. The method according to claim 12 in which the barrier gas is a neutral gas.
 14. The method according to claim 12 in which the barrier gas is maintained by venting the gas from the carburizing furnace.
 15. The method according to claim 12 in which barrier gases are maintained at the entrance to the heating furnace and the exit from the diffusion furnace by venting thereinto the gases for said heating and said diffusion furnaces, respectively.
 16. The method according to claim 15 in which the barrier gas adjacent the carburizing furnace is a neutral gas.
 17. The method according to claim 15 wherein the barrier gas adjacent the carburizing furnace is maintained by venting the gas from said carburizing furnace. 